This disclosure generally relates to magnetorheological fluid compositions, and more particularly, to magnetorheological fluid compositions comprising high aspect ratio magnetizable particles.
Fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field are referred to as Bingham magnetic fluids or magnetorheological fluids. These magnetorheological fluids generally include magnetizable particles dispersed or suspended within a carrier fluid. In the presence of a magnetic field, the magnetizable particles become polarized and are thereby organized into chains of particles within the carrier fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the fluid composition resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. When the flow of the fluid composition is restricted as a result of orientation of the particles into chains, the fluid composition is said to be in its “on state”. The force required to exceed the yield stress is referred to as the “yield strength”. In the absence of a magnetic field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the fluid composition is then correspondingly reduced. The state occupied by the composition in the absence of a magnetic field is referred to as the “off-state”.
Commonly used magnetorheological fluids generally employ magnetizable particles that are symmetrical and have aspect ratios of about 1 to about 1.5. Examples of such particles are spherical particles, ellipsoids, cuboids, or the like. Magnetorheological fluids employing the aforementioned particles are typically used in dampers, clutches, and other torque transfer devices. In these applications, however, the magnetorheological fluid can be subjected to high shear forces causing extreme wear on the magnetizable particles. As a result of this wear, the magnetorheological fluid thickens substantially over time, leading to an increasing off-state viscosity. The increasing off-state viscosity leads to an increase in off-state force experienced by the piston or rotor. This increase in off-state force hampers the freedom of movement of the piston or rotor in certain off-state conditions.
In a magnetorheological device, it is often desirable to maximize the ratio of the on-state force to the off-state force in order to maximize the controllability offered by the device. Since the on-state force is dependent upon the magnitude of the applied magnetic field and the MR fluid composition, the on-state force should remain constant at any given applied magnetic field. If the off-state force increases over time because the off-state viscosity is increasing but the on-state force remains constant, the on-state/off-state ratio will decrease. This decrease in the on-state/off-state ratio results in undesirable minimization of the controllability offered by the device. A more durable magnetorheological fluid that does not thicken over an extended period of time, preferably over the life of the device would be very useful.